OLEDs doped with phosphorescent compounds

ABSTRACT

Organic light emitting devices are disclosed which are comprised of a heterostructure for producing electroluminescence wherein the heterostructure is comprised of an emissive layer containing a phosphorescent dopant compound. For example, the phosphorescent dopant compound may be comprised of platinum octaethylporphine (PtOEP), which is a compound having the chemical structure with the formula:

FIELD OF INVENTION

The present invention is directed to organic light emitting devices(OLEDS) comprised of emissive layers that contain a phosphorescentdopant compound.

BACKGROUND OF THE INVENTION

Organic light emitting devices (OLEDs) are comprised of several organiclayers in which one of the layers is comprised of an organic materialthat can be made to electroluminesce by applying a voltage across thedevice, C. W. Tang et al., Appl. Phys. Lett 51, 913 (1987). CertainOLEDs have been shown to have sufficient brightness, range of color andoperating lifetimes for use as a practical alternative technology toLCD-based full color flat-panel displays (S. R. Forrest, P. E. Burrowsand M. E. Thompson, Laser Focus World, Febraury 1995). Since many of thethin organic films used in such devices are transparent in the visiblespectral region, they allow for the realization of a completely new typeof display pixel in which red (R), green (G), and blue (B) emittingOLEDs are placed in a vertically stacked geometry to provide a simplefabrication process, a small R-G-B pixel size, and a large fill factor,International Patent Application No. PCT/US95/15790.

A transparent OLED (TOLED), which represents a significant step towardrealizing high resolution, independently addressable stacked R-G-Bpixels, was reported in International Patent Application No.PCT/US97/02681 in which the TOLED had greater than 71% transparency whenturned off and emitted light from both top and bottom device surfaceswith high efficiency (approaching 1% quantum efficiency) when the devicewas turned on. The TOLED used transparent indium tin oxide (ITO) as thehole-injecting electrode and a Mg—Ag-ITO electrode layer forelectron-injection. A device was disclosed in which the ITO side of theMg—Ag-ITO electrode layer was used as a hole-injecting contact for asecond, different color-emitting OLED stacked on top of the TOLED. Eachlayer in the stacked OLED (SOLED) was independently addressable andemitted its own characteristic color. This colored emission could betransmitted through the adjacently stacked, transparent, independentlyaddressable, organic layer or layers, the transparent contacts and theglass substrate, thus allowing the device to emit any color that couldbe produced by varying the relative output of the red and bluecolor-emitting layers.

The PCT/US95/15790 application disclosed an integrated SOLED for whichboth intensity and color could be independently varied and controlledwith external power supplies in a color tunable display device. ThePCT/US95/15790 application, thus, illustrates a principle for achievingintegrated, full color pixels that provide high image resolution, whichis made possible by the compact pixel size. Furthermore, relatively lowcost fabrication techniques, as compared with prior art methods, may beutilized for making such devices.

Such devices whose structure is based upon the use of layers of organicoptoelectronic materials generally rely on a common mechanism leading tooptical emission. Typically, this mechanism is based upon the radiativerecombination of a trapped charge. Specifically, OLEDs are comprised ofat least two thin organic layers separating the anode and cathode of thedevice. The material of one of these layers is specifically chosen basedon the material's ability to transport holes, a “hole transportinglayer” (HTL), and the material of the other layer is specificallyselected according to its ability to transport electrons, an “electrontransporting layer” (ETL). With such a construction, the device can beviewed as a diode with a forward bias when the potential applied to theanode is higher than the potential applied to the cathode. Under thesebias conditions, the anode injects holes (positive charge carriers) intothe hole transporting layer, while the cathode injects electrons intothe electron transporting layer. The portion of the luminescent mediumadjacent to the anode thus forms a hole injecting and transporting zonewhile the portion of the luminescent medium adjacent to the cathodeforms an electron injecting and transporting zone. The injected holesand electrons each migrate toward the oppositely charged electrode. Whenan electron and hole localize on the same molecule, a Frenkel exciton isformed. Recombination of this short-lived state may be visualized as anelectron dropping from its conduction potential to a valence band, withrelaxation occurring, under certain conditions, preferentially via aphotoemissive mechanism. Under this view of the mechanism of operationof typical thin-layer organic devices, the electroluminescent layercomprises a luminescence zone receiving mobile charge carriers(electrons and holes) from each electrode.

The materials that produce the electroluminescent emission arefrequently the same materials that function either as the electrontransporting layer or as the hole transporting layer. Such devices inwhich the electron transporting layer or the hole transporting layeralso functions as the emissive layer are referred to as having a singleheterostructure.

Alternatively, the electroluminescent material may be present in aseparate emissive layer between the hole transporting layer and theelectron transporting layer in what is referred to as a doubleheterostructure.

In addition to emissive materials that are present as the predominantcomponent in the charge carrier layer, that is, either in the holetransporting layer or in the electron transporting layer, and thatfunction both as the charge carrier material as well as the emissivematerial, the emissive material may be present in relatively lowconcentrations as a dopant in the charge carrier layer. Whenever adopant is present, the predominant material in the charge carrier layermay be referred to as a host compound or as a receiving compound.Materials that are present as host and dopant are selected so as to havea high level of energy transfer from the host to the dopant material. Inaddition, these materials need to be capable of producing acceptableelectrical properties for the OLED. Furthermore, such host and dopantmaterials are preferably capable of being incorporated into the OLEDusing starting materials that can be readily incorporated into the OLEDby using convenient fabrication techniques, in particular, by usingvacuum-deposition techniques.

It is desirable for OLEDs to be fabricated using materials that provideelectroluminescent emission in a relatively narrow band centered nearselected spectral regions, which correspond to one of the three primarycolors, red, green and blue so that they may be used as a colored layerin an OLED or SOLED. It is also desirable that such compounds be capableof being readily deposited as a thin layer using vacuum depositiontechniques so that they may be readily incorporated into an OLED that isprepared entirely from vacuum-deposited organic materials.

Co-pending U.S. Pat. No. 08/774,087, filed Dec. 23, 1996, now U.S. Pat.No. 6,048,630, is directed to OLEDs containing emitting compounds thatproduce a saturated red emission. The emission layer is comprised of anemitting compound having a chemical structure represented by Formula I:

wherein X is C or N;

R₈, R₉ and R₁₀ are each independently selected from the group consistingof hydrogen, alkyl, substituted alkyl, aryl and substituted aryl;wherein R₉ and R₁₀ may be combined together to form a fused ring;

M₁ is a divalent, trivalent or tetravalent metal; and a, b and c areeach 0 or 1;

wherein, when X is C, then a is 1; when X is N, then a is 0; when c is1, then b is 0; and when b is 1, c is 0.

The examples disclosed in Ser. No. 08/774,087 (now U.S. Pat. No.6,048,630) included an emissive compound of formula I wherein X=C;R₈=phenyl;

R₉=R₁₀=H; c=0; and b=1. This compound has the chemical name5,10,15,20-tetraphenyl-21H,23H-porphine (TPP). OLEDs comprised of theTPP-containing emissive layer produce an emission spectrum comprised oftwo narrow bands that are centered at about 650 and about 713 nm, asshown in FIG. 1. The emission from this device involves fluorescencefrom the TPP dopant. One of the problems with the TPP-doped device isthat the narrow band at 713 nm, which comprises about 40% of theemission, is not within a range that is useful for display applications.A second problem is that TPP-doped OLEDs are very unstable, such thatthe shelf life of such devices is typically very short. It would bedesirable if these two aspects of TPP-doped devices could be improved.The present invention is directed to addressing these problems of priorart devices.

Another aspect of the present invention relates to the fact that, basedon spin statistical arguments, it is generally understood that themajority of the excitons that are produced in an OLED are in anon-emissive triplet electronic state. Formation of such triplet statescan result in a substantial loss of the excitation energy in the OLEDvia radiationless transitions to the ground state. It would be desirableif the total OLED quantum efficiency could be enhanced by utilizing thisenergy transfer pathway through the exciton triplet states, for example,by having the exciton triplet state energy transferred to an emissivematerial. Unfortunately, though it is known that the energy from anexcited triplet state may be efficiently transferred under certaincircumstances to the triplet state of a molecule that phosphoresces, thephosphorescent decay rate is typically not expected to be rapid enoughto be adequate for use in a display device.

The present invention is further directed to OLEDs which also addresssuch problems of prior art devices.

ADVANTAGES AND SUMMARY OF THE INVENTION

The present invention is directed to OLEDs, and a method of fabricatingOLEDs, in which emission from the device is obtained via aphosphorescent decay process wherein the phosphorescent decay rate israpid enough to meet the requirements of a display device.

More specifically, the present invention is directed to OLEDs comprisedof a material that is capable of receiving the energy from an excitonsinglet or triplet state and emitting that energy as phosphorescentradiation.

One of the benefits of the present invention is that the phosphorescentdecay process utilizes exciton triplet state energy that is typicallywasted in an OLED via a radiationless energy transfer and relaxationprocess.

The present invention is further directed to OLEDs that are capable ofproducing a highly saturated red emission. More specifically, OLEDs ofthe present invention are comprised of platinum octaethylporphine(PtOEP), a compound that produces a narrow emission band that peaks near640 nm when the PtOEP is doped in an electron transporting layercomprised of tris-(8-hydroxyquinoline)-aluminum (Alq₃). Such emission isperceived as highly saturated red emission.

Another of the benefits of PtOEP-doped OLEDs is that such OLEDs have astability, when the device is exposed to ambient environmentalconditions for a few days, that is comparable to prior art devices and,in particular, a decidedly greater shelf life stability as compared withTPP-doped devices.

Further objectives and advantages of the present invention will beapparent to those skilled in the art from the detailed description ofthe disclosed invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the electroluminescent (EL) spectrum of TPP-doped OLEDs.

FIG. 2 shows the EL spectra as a function of wavelength at differentvoltages (6, 9, 12 and 15V) for OLEDs having an Alq₃ layer doped with0.6 mol % PtOEP as compared with the EL spectrum of a TPP-doped device(“TPP EL”).

FIG. 3 shows the EL spectra as a function of wavelength at differentvoltages for OLEDs doped with about 6 mol % PtOEP.

FIG. 4 shows the photoluminescent (PL) spectra as a function ofwavelength for different PtOEP concentrations for Alq₃ devices dopedwith PtOEP.

FIG. 5 shows the PL spectra as a function of wavelength at differentexcitation wavelengths for a solution of PtOEP as compared with the ELspectrum of an OLED having an Alq₃ layer doped with 0.6 mol % PtOEP.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention will now be described in detail for specificpreferred embodiments of the invention, it being understood that theseembodiments are intended only as illustrative examples and the inventionis not to be limited thereto.

The present invention is directed to OLEDs in which emission from thedevice is obtained via a phosphorescent decay process wherein thephosphorescent decay rate is rapid enough to meet the requirements of adisplay device. As a representative embodiment of the present inventionthe emission layer is comprised of an emitting compound having astructure represented by Formula I:

wherein M=Pt; a=1; b=0; c=1; X=C; and R₈=H; and

R₉=R₁₀=Et (ethyl). In particular, this compound, platinumoctaethylporphine (PtOEP), has the chemical structure of formula II:

The advantage of selecting a dopant compound such as PtOEP as theemissive material of an OLED is based, inter alia, on two particularfacts. First, the photoluminescent quantum yield for this molecule issignificantly greater than TPP, PtOEP having a photoluminescent quantumyield of greater than 50%, and as high as 90% in the solid state, andTPP having a photoluminescent quantum yield of only about 10%. Theimproved photoluminescent quantum yield makes it possible to fabricateOLEDs with increased efficiencies. A second advantage that is offered byselecting a phosphorescent compound such as PtOEP is that the emissionfrom such a molecule comes from a triplet state. A molecule that iscapable of being excited to a triplet state provides the possibility ofhaving the energy transferred from the non-emissive exciton tripletstate to a triplet state that is capable of radiatively emitting thisenergy as phosphorescent radiation. Though phosphorescence, which refersto radiation that comes from a triplet state, typically occurs at a muchslower rate than fluorescence, which refers to radiation from a singletstate, the phosphorescence from a compound such as PtOEP is,nevertheless, sufficiently rapid to satisfy the requirements of certaindisplay devices. In particular, a compound such as PtOEP, which has alifetime of about 7 μsec when used as the dopant in an Alq₃ layer, maybe used in passive matrix displays that require a switching time of notfaster than about 10 μsec or in an active matrix display for which theswitching time only needs to be about 10 msec.

As a representative embodiment of the present invention, the PtOEP maybe doped into the Alq₃ layer of an ITO/TPD/Alq₃/Mg—Ag OLED. The behaviorof such PtOEP-doped OLEDs is very different from OLEDs prepared with TPPdopants. At doping levels greater than 0.5 mol % TPP, the emission fromthe OLED is exclusively from the TPP. In contrast, at low to moderatedoping levels of PtOEP in Alq₃, the emission is dominated by PtOEPemission at low voltage, but as the voltage is increased, Alq₃ emissionappears. At moderately high voltages (e.g., 15 V) the majority of theemission comes from Alq₃. The EL spectra for a 0.6 mol % PtOEP dopedOLED are given in FIG. 2. The spectra for 1.3 mol % PtOEP have about thesame shape as those shown for the 0.6 mol % device. The shape of thespectra of an OLED prepared with 6 mol % PtOEP are shown in FIG. 3. Asthe voltage is increased, the intensity of the red emission increasessignificantly, but a contribution from Alq₃ emission is not observed,even at a high voltage.

While the present invention is not limited by the theory of how itworks, it is believed that the explanation for the increase in Alq₃emission as the voltage is increased is related to the differentlifetimes for photoluminescence for Alq₃ and PtOEP. The PL lifetime forAlq₃ is about 13 nsec (nanoseconds) in both the solid state and insolution, whereas the PL lifetime of PtOEP varies from about 10 to about100 μsec (microseconds) depending on the medium. If the voltage appliedto the PtOEP-doped device is kept low, the number of excitonstransferred to PtOEP is small enough such that the excited PtOEPmolecules can relax at a sufficient rate relative to the Alq₃ excitoncreation rate, with the result that there are always enough dopantmolecules for energy transfer from the Alq₃. As the voltage isincreased, the available PtOEP dopant molecules become saturated andcannot relax fast enough to keep up with the rate at which the excitonsare being created in the Alq₃. In this higher voltage regime, some ofthe Alq₃ excitons relax by radiative emission before the excitationenergy can be transferred to the PtOEP molecules. At 6 mol % PtOEP,enough dopant is present to trap all of the excitons, but the higherdoping levels lead to decreased overall efficiency.

This explanation is further supported by the results shown in FIG. 4,which show the PL spectra as a function of wavelength at differentdoping levels for PtOEP-doped Alq₃ devices. At the lowest doping levelsof 0.6 mol %, a large emission band characteristic of Alq₃ is observed,whereas for the high 6 mol % PtOEP-doping level, there appears to besufficient PtOEP present to capture all the exciton energy from theAlq₃.

The emission from the PtOEP-based OLEDs is very narrow and centered at645 nm. This narrow band, which corresponds to saturated red emission,has a full width at half maximum of about 30 nm. A comparison of the PLspectra as a function of wavelength at different excitation wavelengthsfor PtOEP in solution, as shown in FIG. 5, with the EL spectrum of aPtOEP-doped Alq₃ OLED, shows that a PtOEP-doped OLED selectivelyproduces the narrow band of emission from PtOEP that is centered with apeak at about 645 nm. This narrow, highly saturated red emission isproduced, with almost the total absence of the other PtOEP peaks, eventhough a comparison of the PL excitation spectrum of PtOEP with thebroad emission band from the Alq₃ might lead one to expect additionalbands centered at about 620 and about 685 nm, as is observed for the PLspectra of PtOEP in solution.

The net result is that the emission from a PtOEP-doped device issignificantly better, with respect to the saturated red emission, thanthat of a TPP-doped device since there is no long wavelength tail orpeak above 700 nm. The quantum efficiencies for these devices, which areexternal quantum yields, are listed in Table 1. In each case, theefficiency is listed along with that of a reference device(ITO/TPD/Alq₃/Mg—Ag) prepared in parallel with the doped device. At lowdrive voltages, the efficiencies of the doped devices are superior,while at higher voltages the undoped device has a higher efficiency.These results show that PtOEP-doped devices are capable of performingwith efficiencies comparable to prior art Alq₃-doped devices.

The shelf lives of PtOEP devices that were exposed to ambientenvironmental conditions for a few days were observed to be comparableto undoped Alq₃ devices and decidedly superior to devices prepared withTPP as the dopant.

TABLE 1 External quantum efficiencies for PtOEP-doped Alq₃ OLEDs ascompared with an undoped Alq₃ OLED reference. Conc. Voltage n(doped)n(ref.) 0.6 mol % low volt (6-7 V) 0.2%  0.15% high volt (11-12 V) 0.07%0.18% 1.3 mol % low volt (6-7 V) 0.2%  0.11% high volt (11-12 V) 0.11%0.24%   6 mol % low volt (6-7 V) 0.14% 0.17% high volt (11-12 V) 0.07%0.2% 

Such OLEDs may be used, for example, in passive matrix flat paneldisplays having a switching time not faster than about 10 μsec, inactive matrix displays for which the switching time only needs to beabout 10 msec, or in low resolution display applications. Thephosphorescent compounds may be selected, for example, from thosephosphorescent compounds which have the chemical structure of formula I:

wherein X is C or N;

R₈, R₉ and R₁₀ are each independently selected from the group consistingof hydrogen, alkyl, substituted alkyl, aryl and substituted aryl;

R₉ and R₁₀ may be combined together to form a fused ring;

M₁ is a divalent, trivalent or tetravalent metal; and a, b and c areeach 0 or 1;

wherein, when X is C, then a is 1; when X is N, then a is 0;

when c is 1, then b is 0; and when b is 1, c is 0.

The phosphorescent compounds may also be selected, as another example,from phosphorescent porphyrin compounds, which may be partly or fullyhydrogenated.

In addition to selecting phosphorescent compounds according to theirphosphorescent lifetimes, which for certain applications may meanselecting compounds having a phosphorescent lifetime not longer thanabout 10 μsec, the phosphorescent compounds may be selected according totheir ability to effectively capture the exciton triplet energy from acharge carrier material and then to emit that excitation energy asphosphorescence in a narrow emission band corresponding to a highlysaturated color, such as demonstrated by PtOEP in an Alq₃-based OLED.

A dopant capable of shifting the emission wavelength of an emissivelayer comprised only of a host compound is added to the host compound inan amount effective to shift the wavelength of emission so that the LEDdevice preferably emits light that is perceived by the human eye to beclose to one of the primary colors. Although it is recognized thatcharacterization of color perception is a subjective exercise, aquantitative chromaticity scale has been developed by the CommissionInternationale de l'Eclairage (International Commission ofIllumination), otherwise known as the CIE standard. According to thisstandard, a saturated color may be represented by a single point, withspecific quantitative coordinates according to the defined axes of thechromaticity scale. It will be appreciated by one of skill in the artthat such a single point on the CIE scale would represent a standard ora goal that, in practical terms, is difficult, but fortunately,unnecessary, to attain.

In the preferred embodiments of the present invention in which the OLEDpredominantly produces a primary color, the dopant is incorporated intoa host compound so that the OLED emits light that is perceived by thehuman eye to be close to a saturated primary color. Through the practiceof the present invention, it is intended that OLEDs be constructed whichcan be characterized by an emission that is close to an absolute (orsaturated) chromaticity value, as that would be defined by the CIEscale. Furthermore, LED's utilizing the materials of the presentinvention are also intended to be capable of a display brightness thatcan be in excess of 100 cd/m² although somewhat lower values, perhaps aslow as 10 cd/m², may be acceptable in certain cases.

The host compounds as defined herein are compounds which can be dopedwith dopants that emit light with the desired spectral characteristics.Such compounds include, but are not limited to, the emitting compoundsand host compounds as described in U.S. patent application Ser. No.08/693,359, filed Aug. 6, 1996, incorporated herein by reference. Theterm “host” is used to refer to the compound in the emissive layer thatfunctions as the component, that is, “receiving compound” which receivesthe hole/electron recombination energy and then by anemission/absorption energy transfer process, transfers that excitationenergy to the dopant compound, which is typically present in much lowerconcentrations. The dopant may then relax to an excited state having aslightly lower energy level, which preferentially radiates all of itsenergy as luminescent emission in a desired spectral region. A dopantthat radiates 100% of the dopant's excited state excitation energy issaid to have a quantum efficiency of 100%. For host/dopantconcentrations which are to be used in a color tunable SOLED, preferablymost, if not all, of the host's excitation energy is transferred to thedopant which in turn radiates, perhaps from a lower energy level, butwith a high quantum efficiency, to produce visible radiation having adesired chromaticity. The present invention is directed towardphosphorescent compounds that are intended to serve as dopants whichsatisfy these demanding energy transfer requirements.

As the term host compound is used herein, it will be appreciated thatsuch compounds can be found in an electron transporting/emissive layeror a hole transporting/emissive layer of a single heterostructure OLEDdevice or in the separate emissive layer of a double heterostructuredevice.

As will be recognized by one of skill in the art, use of the dopantspecies such as disclosed herein makes it possible to extend not onlythe range of colors emitted by the OLED, but also to extend the range ofpossible candidate species for host and/or dopant compounds.Accordingly, for effective host/dopant systems, although the hostcompound can have a strong emission in a region of the spectrum wherethe dopant species strongly absorbs light, the host species preferablydoes not have an emission band in a region where the dopant also emitsstrongly. In structures where the host compound also functions as acharge carrier, then additional criteria such as redox potential for thespecies also becomes a consideration. In general, however, the spectralcharacteristics of the host and dopant species are the most importantcriteria.

The amount of dopant that is present is that amount which is sufficientto shift the emission wavelength of the host material as close aspossible to a saturated primary color, as that would be definedaccording to the CIE scale. Typically, the effective amount is fromabout 0.01 to 10.0 mol %, based on the emitting layer. The primarycriterion for determining an appropriate doping level is the level whichis effective for achieving an emission with the appropriate spectralcharacteristics. By way of example, and without limitation, if theamount of dopant species is at too low a level, then emission from thedevice will also comprise a component of light from the host compounditself, which will be at shorter wavelengths than the desired emissionform the dopant species. In contrast, if the level of dopant is toohigh, emission efficiencies could be adversely affected byself-quenching, a net non-emissive mechanism. Alternatively, too highlevels of the dopant species could also adversely affect the hole orelectron transporting properties of the host material.

The OLEDs of the present invention are comprised of a heterostructurefor producing electroluminescence which may be fabricated as a singleheterostructure or as a double heterostructure. The materials, methodsand apparatus for preparing the organic thin films of a single or doubleheterostructure are disclosed, for example, in U.S. Pat. No. 5,554,220,which is incorporated herein in its entirety by reference. As usedherein, the term “heterostructure for producing electroluminescence”refers to a heterostructure that includes, for a single heterostructure,in sequence, a hole injecting anode layer, a hole transporting layer, anelectron transporting layer, and a cathode layer. An additional layer orlayers may be present between one or more of the sequential pairs ofthese layers. For example, for a double heterostructure, a separateemissive layer is included between the hole transporting layer and theelectron transporting layer. This separate emissive layer may becharacterized as being a “thin luminescent layer.” Alternatively, or inaddition, a hole injection enhancement layer may be present between theanode layer and the hole transporting layer and/or an electron injectingand interface layer may be present between the cathode layer and theelectron transporting layer.

Either the anode layer or the cathode layer may be in contact with asubstrate and each electrode is connected to electrical contacts whichare capable of delivering a voltage across the device causing it toproduce electroluminescence from an electron transporting layer, a holetransporting layer or a separate emissive layer. If the cathode layer isdeposited on the substrate, the device may be referred to as having aninverted OLED (IOLED) structure. An inverted structure may also bereferred to as an “OILED” structure. If the heterostructure forproducing electroluminescence is included as part of a stacked OLED(SOLED), one or both of the electrodes of an individual heterostructuremay be in contact with an electrode of an adjacent heterostructure.Alternatively, dependent on the circuitry used to drive the SOLED, aninsulating layer may be provided between the adjacent electrodes of theOLEDs in the stack.

The single or double heterostructures as referred to herein are intendedsolely as examples for showing how an OLED embodying the presentinvention may be fabricated without in any way intending the inventionto be limited to the particular materials or sequence for making thelayers shown. For example, a single heterostructure typically includes asubstrate which may be opaque or transparent, rigid or flexible, and/orplastic, metal or glass; a first electrode, which is typically a highwork function, hole-injecting anode layer, for example, an indium tinoxide (ITO) anode layer; a hole transporting layer; an electrontransporting layer; and a second electrode layer, for example, a lowwork function, electron-injecting, metal cathode layer of amagnesium-silver alloy, (Mg:Ag) or of a lithium-aluminum alloy, (Li:Al).Alternatively, as disclosed in the recently filed application entitled“A Highly Transparent Organic Light Emitting Device Employing aNon-Metallic Cathode” filed Nov. 5, 1997, which is incorporated in itsentirety by reference, the cathode may be a non-metallic material such aITO, the term “non-metallic” being used to embrace still othertransparent conducting inorganic layers, as well as materials comprisedof metals that may be present as one of the elements in a chemicalcompound, for example, as an oxide. However, the term “non-metallic”does not embrace materials comprised predominantly of the free metal nordoes the term embrace metal alloys.

Materials that may be used as the substrate in a representativeembodiment of the present invention include, in particular, glass,transparent polymer such as polyester, sapphire or quartz, orsubstantially any other material that may be used as the substrate of anOLED.

Materials that may be used as the hole-injecting anode layer in arepresentative embodiment of the present invention include, inparticular, ITO, Zn—In—SnO₂ or SbO₂, or substantially any other materialthat may be used as the hole-injecting anode layer of an OLED.

Materials that may be used in the hole transporting layer in arepresentative embodiment of the present invention include, inparticular,N,N′-diphenyl-N,N′-bis(3-methylpheny)1-1′biphenyl-4,4′diamine (TPD),4,4′-bis[N-(1-naphthyl)-N-phenyl-amino]biphenyl (α-NPD) or4,4′-bis[N-(2-naphthyl)-N-phenyl-amino]biphenyl (β-NPD).

Materials that may be used as the electron transporting layer include,in particular, tris-(8-hydroxyquinoline)-aluminum (Alq₃) and carbazole.

Materials that may be used as the separate emissive layer, if present,include, in particular, dye-doped Alq₃, or substantially any othermaterial that may be used as the separate emissive layer of an OLED.

The insulating layer, if present, may be comprised of an insulatingmaterial such as SiO₂, SiN_(x) or AlO₂, or substantially any othermaterial that may be used as the insulating material of an OLED, whichmay be deposited by a variety of processes such as plasma enhancedchemical vapor deposition (PECVD), electron beam, etc.

The hole injecting enhancement layer may in some cases be comprised ofthe same material, CuPc, as is used in the electron injecting andinterface layer. In each case, the CuPc layer may be in direct contactwith an ITO electrode, with the distinction between the two CuPc layersbeing that in one case the CuPc layer is in contact with an ITO layerthat functions as an anode and in the other case the ITO layer functionsas a cathode. In each case, the CuPc layer functions as a charge carrierand interface layer. On the one hand when in contact with the ITO anode,the CuPc layer assists in injecting and transporting holes from theanode to a hole transporting layer, and on the other hand when incontact with the ITO cathode, the CuPc layer assists in injecting andtransporting electrons from the cathode to an electron transportinglayer. The term “electron injecting interface layer” is used to refer tothis layer that is present between and in contact with the cathode layerand the electron transporting layer of the heterostructure. The CuPclayer, in each case, may also function as a protection layer thatprotects any underlying organic layers, if present, from damage duringthe ITO deposition process. The protection layer may also be comprisedof others materials such a 3,4,9,10-perylenetetra-carboxylic dianhydride(PTCDA). Whenever the ITO layer is present as the electrode in a SOLEDstructure, opposite faces of the ITO may function as an anode andcathode, respectively.

The OLEDs of the present invention have the advantage that they can befabricated entirely from vacuum-deposited molecular organic materials asdistinct, for example, from OLEDs in which some of the layers arecomprised of polymeric materials, which cannot be readily depositedusing vacuum deposition techniques. A vacuum-deposited material is onewhich can be deposited in a vacuum typically having a backgroundpressure less than one atmosphere, preferably about 10⁻⁵ to about 10⁻¹¹torr for vacuum deposition, or about 50 torr to about 10⁻⁵ torr forvapor deposition.

Although not limited to the thickness ranges recited herein, thesubstrate may be as thin as 10 μ, if present as a flexible plastic ormetal foil substrate, such as aluminum foil, or substantially thicker ifpresent as a rigid, transparent or opaque, substrate or if the substrateis comprised of a silicon-based display driver; the ITO anode layer maybe from about 500 Å (1 Å=10⁻⁸ cm) to greater than about 4000 Å thick;the hole transporting layer from about 50 Å to greater than about 1000 Åthick; the separate emissive layer of a double heterostructure, ifpresent, from about 50 Å to about 200 Å thick; the electron transportinglayer from about 50 Å to about 1000 Å thick; and the metal cathode layerfrom about 50 Å to greater than about 100 Å thick, or substantiallythicker if the cathode layer includes a protective silver layer and isopaque.

Thus, while there may be substantial variation in the type, number,thickness and order of the layers that are present, dependent on whetherthe device includes a single heterostructure or a doubleheterostructure, whether the device is a SOLED or a single OLED, whetherthe device is a TOLED or an IOLED, whether the OLED is intended toproduce emission in a preferred spectral region, or whether still otherdesign variations are used, the present invention is directed to thosedevices in which the OLED is comprised of a heterostructure forproducing electroluminescence wherein the heterostructure is comprisedof an emissive layer containing a phosphorescent compound.

The subject invention as disclosed herein may be used in conjunctionwith co-pending applications: “High Reliability, High Efficiency,Integratable Organic Light Emitting Devices and Methods of ProducingSame”, Ser. No. 08/774,119 (filed Dec. 23, 1996), now U.S. Pat. No.6,046,543; “Novel Materials for Multicolor Light Emitting Diodes”, Ser.No. 08/850,264 (filed May 2, 1997), now U.S. Pat. No. 6,045,930;“Electron Transporting and Light Emitting Layers Based on Organic FreeRadicals”, Ser. No. 08/774,120 (filed Dec. 23, 1996), now U.S. Pat. No.5,811,833; “Multicolor Display Devices”, Ser. No. 08/772,333 (filed Dec.23, 1996), now U.S. Pat. No. 6,013,982; “Red-Emitting Organic LightEmitting Devices (OLED's)”, Ser. No. 08/774,087 (filed Dec. 23, 1996),now U.S. Pat. No. 6,048,630; “Driving Circuit For Stacked Organic LightEmitting Devices”, Ser. No. 08/792,050 (filed Feb. 3, 1997)(U.S. Pat.No. 5,757,139); “High Efficiency Organic Light Emitting DeviceStructures”, Ser. No. 08/772,332 (filed Dec. 23, 1996)(U.S. Pat. No.5,834,893); “Vacuum Deposited, Non-Polymeric Flexible Organic LightEmitting Devices”, Ser. No. 08/789,319 (filed Jan. 23, 1997)(U.S. Pat.No. 5,844,363); “Displays Having Mesa Pixel Configuration”, Ser. No.08/794,595 (filed Feb. 3, 1997), now U.S. Pat. No. 6,091,195; “StackedOrganic Light Emitting Devices”, Ser. No. 08/792,046 (filed Feb. 3,1997)(U.S. Pat. No. 5,927,280); “High Contrast Transparent Organic LightEmitting Device Display”, Ser. No. 08/821,380 (filed Mar. 20, 1997), nowU.S. Pat. No. 5,986,401; “Organic Light Emitting Devices Containing AMetal Complex of 5-Hydroxy-Quinoxaline as A Host Material”, Ser. No.08/838,099 (filed Apr. 15, 1997)(U.S. Pat. No. 5,861,219); “LightEmitting Devices Having High Brightness”, Ser. No. 08/844,353 (filedApr. 18, 1997), now U.S. Pat. No. 6,125,226; “Organic SemiconductorLaser”, Ser. No. 08/859,468 (filed May 19, 1997), now U.S. Pat. No.6,111,902; “Saturated Full Color Stacked Organic Light EmittingDevices”, Ser. No. 08/858,994 (filed on May 20, 1997), now U.S. Pat. No.5,932,895; “An Organic Light Emitting Device Containing a Hole InjectionEnhancement Layer”, Ser. No. 08/865,491 (filed May 29, 1997), now U.S.Pat. No. 5,998,803; “Plasma Treatment of Conductive Layers”,PCT/US97/10252, (filed Jun. 12, 1997); “Patterning of Thin Films for theFabrication of Organic Multi-color Displays”, PCT/US97/10289, (filedJun. 12, 1997); “OLEDs Containing Thermally Stable Asymmetric ChargeCarrier Materials”, Ser. No. 08/929,029, filed Sep. 8, 1997, now U.S.Pat. No. 6,242,115; “Light Emitting Device with Stack of OLEDS andPhosphor Downconverter”, Ser. No. 08/925,403, (filed Sep. 9, 1997)(U.S.Pat. No. 5,874,803); “An Improved Method for Depositing Indium Tin OxideLayers in Organic Light Emitting Devices”, Ser. No. 08/928,800 (filedSep. 12, 1997), now U.S. Pat. No. 5,981,306; “Azlactone-Related Dopantsin the Emissive Layer of an Oled” (filed Oct. 9, 1997), Ser. No.08/948,130, “A Highly Transparent Organic Light Emitting DeviceEmploying A Non-Metallic Cathode”, (filed Nov. 3, 1997) now U.S. Pat.No. 6,030,715; (Provisional),“A Highly Transparent Organic LightEmitting Device Employing a Non-Metallic Cathode”, (filed Nov. 5, 1997),(Ser. No. 08/964,863), “Low Pressure Vapor Phase Deposition of OrganicThin Films” (filed Nov. 17, 1997), (Ser. No. 08/972,156), “Method ofFabricating and Patterning Oleds”, (filed Nov. 24, 1997), (Ser. No.08/977,205), “Method for Deposition and Patterning of Organic ThinFilm”, (filed Nov. 24, 1997) now U.S. Pat. No. 6,013,538; (Ser. No.08/976,666), now U.S. Pat. No. 5,953,587; each co-pending applicationbeing incorporated herein by reference in its entirety. The subjectinvention may also be used in conjunction with the subject matter ofeach of co-pending U.S. patent application Ser. No. 08/354,674 (U.S.Pat. No. 5,707,745), U.S. Ser. No. 08/613,207 (U.S. Pat. No. 5,703,436),U.S. Ser. No. 08/632,322 (U.S. Pat. No. 5,757,026) and U.S. Ser. No.08/693,359 and provisional patent application Serial No. 60/010,013(U.S. Ser. No. 08/779,141, filed Jan. 6, 1997, Allowed) U.S. Ser. No.60/024,001 (U.S. Ser. No. 08/789,319, filed Jan. 23, 1997, U.S. Pat. No.5,844,363) U.S. Ser. No. 60/025,501 (U.S. Ser. No. 08/844,353, filedApr. 18, 1997), U.S. Ser. No. 60/046,061 (U.S. Ser. No. 09/010,594,filed Jan. 22, 1998) and U.S. Ser. No. 60/053,176, each of which is alsoincorporated herein by reference in its entirety.

The materials that may be used as the substrate, the hole-injectinganode layer, the hole transporting layer, the electron transportinglayer, the electron-injecting, metal cathode layer or theelectron-injecting, non-metallic cathode layer, the protection layer, ifpresent, the separate emissive layer, if present, or the insulatinglayer, if present, include the materials as disclosed in theseco-pending applications.

The OLED of the present invention may be used in substantially any typeof device which is comprised of an OLED, for example, in OLEDs that areincorporated into a larger display, a vehicle, a computer, a television,a printer, a large area wall, theater or stadium screen, a billboard ora sign.

This invention will now be described in detail with respect to showinghow certain specific representative embodiments thereof can be made, thematerials, apparatus and process steps being understood as examples thatare intended to be illustrative only. In particular, the invention isnot intended to be limited to the methods, materials, conditions,process parameters, apparatus and the like specifically recited herein.

AN EXAMPLE OF THE INVENTION

The procedures that were used for fabrication of Organic Light-EmittingDevices (OLEDs) were as follows:

The hole transporting material TPD and the electron transportingmaterial Alq₃ were synthesized according to literature procedures, andwere sublimed before use. The dopant PtOEP was purchased from PorphyrinProducts, Inc., Logan, UT, and was used as received.

OLEDs were prepared using the following procedures: The ITO/Borosilicatesubstrates (100Ω/square) were cleaned by sonicating with detergent forfive minutes followed by rinsing with deionized water. They were thentreated twice in boiling 1,1,1-trichloroethane for two minutes. Thesubstrates were then sonicated twice with acetone for two minutes andtwice with methanol for two minutes.

The background pressure prior to deposition was normally 7×10⁻⁷ torr orlower and the pressure during the deposition was around 5×10⁻⁷ to1.1×10⁻⁶ torr.

All the chemicals were resistively heated in various tantalum boats. TPDwas first deposited at a rate from one to four Å/s. The thickness wastypically controlled at 300 Å.

The electron transporting layer Alq₃ was doped with PtOEP.

Typically, the dopant was first vaporized with the substrates covered.After the rate of the dopant was stabilized, the host material wasvaporized to the certain rate. The cover over the substrates was thenopened and the host and guest were deposited at the desiredconcentration. The rate of dopant was normally 0.1-0.2 Å/s. The totalthickness of this layer was controlled at about 450 Å.

The substrates were removed from the deposition system and masks wereput directly on the substrates. The masks were made of stainless steelsheet and contain holes with diameters of 0.25, 0.5, 0.75 and 1.0 mm.The substrates were then put back into vacuum for further coating.

Magnesium and silver were co-deposited at a rate normally of 2.6 Å/s.The ratio of Mg:Ag varied from 7:1 to 12:1. The thickness of this layerwas typically 500 Å. Finally, 1000 Å Ag was deposited at the ratebetween one to four Å/s.

The devices were characterized within five hours of fabrication.Typically electroluminescent spectra, I-V curves, and quantum yieldswere measured from direct front.

What is claimed is:
 1. An organic electroluminescent emissive layercomprising a charge carrying, host material and a phosphorescentmaterial, wherein the phosphorescent material is present in a lowerconcentration than the charge carrying, host material and thephosphorescent material emits phosphorescent radiation from a tripletmolecular excited state when a voltage is applied across theelectroluminescent emissive layer.
 2. The organic electroluminescentlayer of claim 1 wherein the phosphorescent material is present at aconcentration from about 0.01 to 10.0 mol % based on theelectroluminescent layer.
 3. The organic electroluminescent layer ofclaim 1 wherein the charge carrying, host material is a holetransporting material.
 4. The organic electroluminescent layer of claim1 wherein the charge carrying, host material is an electron transportingmaterial.
 5. The organic electroluminescent layer of claim 1 wherein thephosphorescent material has a phosphorescent lifetime not longer thanabout 10 microseconds.
 6. The organic electroluminescent layer of claim1 wherein the phosphorescent lifetime from about 10 to about 100microseconds.
 7. A device for producing electroluminescence comprisingan organic light emitting device including a phosphorescent material anda charge carrying host material, wherein the phosphorescent materialemits phosphorescent radiation from a triplet molecular excited statewhen a voltage is applied across the organic light emitting device. 8.The device of claim 7 wherein the phosphorescent material has aphosphorescent lifetime not longer than about 10 microseconds.
 9. Thedevice of claim 7 wherein the phosphorescent material has aphosphorescent lifetime from about 10 to about 100 microseconds.
 10. Thedevice of claim 7 wherein the phosphorescent radiation produces aprimary color.
 11. The device of claim 10 wherein the primary color isred.
 12. The device of claim 10 wherein the primary color is green. 13.The device of claim 10 wherein the primary color is blue.
 14. A devicefor producing electroluminescence comprising an organic light emittingdevice including an emissive layer that emits phosphorescent radiationfrom a triplet molecular excited state when a voltage is applied acrossthe device, wherein the emissive layer includes a host material and aphosphorescent material, the phosphorescent material having the chemicalstructure of formula I:

wherein X is C or N; R₈, R₉ and R₁₀ are each independently selected fromthe group consisting of hydrogen, alkyl, substituted alkyl, aryl andsubstituted aryl; R₉ and R₁₀ may be combined together to form a fusedring; M₁ is a divalent, trivalent or tetravalent metal; and a, b and care each 0 or 1; wherein, when X is C, then a is 1; when X is N, then ais 0; when c is 1, then b is 0; and when b is 1, c is
 0. 15. The deviceof claim 14 wherein the phosphorescent material is present at aconcentration from about 0.01 to 10.0 mol % based on the emissive layer.16. The device of claim 14 wherein M₁ is a tetravalent metal.
 17. Thedevice of claim 14 wherein M₁ is Pt.
 18. The device of claim 14 whereinthe phosphorescent material is platinum octaethylporphine and has thechemical structure with the formula, wherein Et is an ethyl group:


19. The device of claim 14 wherein the phosphorescent material has aphosphorescent lifetime not longer than about 10 microseconds.
 20. Thedevice of claim 14 wherein the phosphorescent material has aphosphorescent lifetime from about 10 to about 100 microseconds.
 21. Thedevice of claim 14 wherein the phosphorescent radiation produces aprimary color.
 22. The device of claim 21 wherein the primary color isred.
 23. The device of claim 21 wherein the primary color is green. 24.The device of claim 21 wherein the primary color is blue.
 25. The deviceof claim 14 wherein the phosphorescent radiation peaks at about 640 nm.26. A device for producing electroluminescence comprising an organiclight emitting device having an emissive layer comprised of a chargecarrying, host material and a phosphorescent material, wherein thephosphorescent material is present at a lower concentration than thecharge carrying, host material, and wherein the phosphorescent materialemits phosphorescent radiation from a triplet molecular excited statewhen a voltage is applied across the organic light emitting device. 27.The device of claim 26 wherein the emissive layer is vapor-deposited.28. The device of claim 26 wherein the emissive layer isvacuum-deposited.
 29. The device of claim 26 wherein the phosphorescentmaterial is present at a concentration from about 0.01 to 10.0 mol %based on the emissive layer.
 30. The device of claim 26 wherein thecharge carrying, host material is a hole transporting material.
 31. Thedevice of claim 26 wherein the charge carrying, host material is anelectron transporting material.
 32. The device of claim 26 wherein thephosphorescent material has a phosphorescent lifetime not longer thanabout 10 microseconds.
 33. The device of claim 26 wherein thephosphorescent material has a phosphorescent lifetime from about 10 toabout 100 microseconds.
 34. The device of claim 26 wherein the organiclight emitting device comprises a heterostructure.
 35. The device ofclaim 34 wherein the heterostructure comprises, in sequence, an anodelayer, a hole transporting layer, an electron transporting layer, and acathode layer, wherein the emissive layer is the electron transportinglayer.
 36. The device of claim 35 wherein the electron transportinglayer comprises tris-(8-hydroxyquinoline)-aluminum.
 37. The device ofclaim 35 wherein the hole transporting layer comprisesN,N′-diphenyl-N,N′-bis(3-methylpheny)1-1′biphenyl-4,4′diamine.
 38. Thedevice of claim 35 wherein a substrate is present on the anode side ofthe light emitting device.
 39. The device of claim 35 wherein asubstrate is present on the cathode side of the light emitting device.40. The device of claim 34 wherein the heterostructure comprises, insequence, an anode layer, a hole transporting layer, an electrontransporting layer, and a cathode layer, wherein the emissive layer isthe hole transporting layer.
 41. The device of claim 34 wherein theheterostructure comprises, in sequence, an anode layer, a holetransporting layer, an electron transporting layer, and a cathode layer,wherein the emissive layer is between the electron transporting layerand the hole transporting layer.
 42. The device of claim 26 wherein thephosphorescent radiation produces a primary color.
 43. The device ofclaim 42 wherein the primary color is red.
 44. The device of claim 42wherein the primary color is green.
 45. The device of claim 42 whereinthe primary color is blue.
 46. A stacked organic light emitting devicecomprising: a first organic light emitting device for producing light ofa first color; and a second organic light emitting device for producinglight of a second color; the second organic light emitting device beingstacked on the first organic light emitting device; wherein at least oneof the organic light emitting devices includes a phosphorescent materialin a charge carrying, host material, and wherein the phosphorescentmaterial emits phosphorescent radiation from a triplet molecular excitedstate when a voltage is applied across the at least one organic lightemitting device.
 47. A stacked organic light emitting device comprisinga first organic light emitting device for producing light of a firstcolor; and a second organic light emitting device for producing light ofa second color; the second organic light emitting device being stackedon the first organic light emitting device; wherein at least one of theorganic light emitting devices includes an emissive layer that emitsphosphorescent radiation from a triplet molecular excited state when avoltage is applied across the device, wherein the emissive layerincludes a host material and a phosphorescent material, thephosphorescent material having the chemical structure of formula I:

wherein X is C or N; R₈, R₉ and R₁₀ are each independently selected fromthe group consisting of hydrogen, alkyl, substituted alkyl, aryl andsubstituted aryl; R₉ and R₁₀ may be combined together to form a fusedring; M₁ is a divalent, trivalent or tetravalent metal; and a, b and care each 0 or 1; wherein, when X is C, then a is 1; when X is N, then ais 0; when c is 1, then b is 0; and when b is 1, c is
 0. 48. The stackedorganic light emitting device of claim 47 wherein M₁ is a tetravalentmetal.
 49. The stacked organic light emitting device of claim 47 whereinM₁ is Pt.
 50. The stacked organic light emitting organic light emittingdevice of claim 47 wherein the phosphorescent material is platinumoctaethylporphine and has the chemical structure with the formula,wherein Et is an ethyl group:


51. A method of manufacturing a light emitting device comprising:fabricating an organic light emitting device including an emissivelayer, wherein the fabrication process includes the step of depositing aphosphorescent material and a charge carrying, host material to form theemissive layer, wherein the phosphorescent material emits phosphorescentradiation from a triplet molecular excited state when a voltage isapplied across the device.
 52. The method of claim 51 wherein theemissive layer is vapor-deposited.
 53. The method of claim 51 whereinthe emissive layer is vacuum-deposited.
 54. The method of claim 51wherein the phosphorescent material has a phosphorescent lifetime notlonger than about 10 microseconds.
 55. The method of claim 51 whereinthe phosphorescent material has a phosphorescent lifetime from about 10to about 100 microseconds.
 56. A method of manufacturing a lightemitting device comprising: fabricating an organic light emitting deviceincluding an emissive layer; wherein the fabrication process includesthe step of depositing a charge carrying, host material together with aphosphorescent material to form the emissive layer, wherein thephosphorescent material has a lower concentration in the emissive layerthan the charge carrying material and the phosphorescent material emitsphosphorescent radiation from a triplet molecular excited state when avoltage is applied across the organic light emitting device.
 57. Themethod of claim 56 wherein the step of depositing the charge carrying,host material together with the phosphorescent material to form theemissive layer comprises vapor depositing the charge carrying, hostmaterial together with the phosphorescent material to form the emissivelayer.
 58. The method of claim 56 wherein the step of depositing thecharge carrying, host material together with the phosphorescent materialto form the emissive layer comprises vacuum depositing the chargecarrying, host material together with the phosphorescent material toform the emissive layer.
 59. The method of claim 56 wherein thephosphorescent material is present at a concentration from about 0.01 to10.0 mol % based on the emissive layer.
 60. The method of claim 56wherein the charge carrying, host material is a hole transportingmaterial.
 61. The method of claim 56 wherein the charge carrying, hostmaterial is an electron transporting material.
 62. The method of claim56 wherein the phosphorescent material has a phosphorescent lifetime notlonger than about 10 microseconds.
 63. The method of claim 56 whereinthe phosphorescent material has a phosphorescent material has aphosphorescent lifetime from about 10 to about 100 microseconds.